Example: A rectangle
has its vertices on the x-axis, on the y-axis, at the
origin, and somewhere on the graph y=4−x2 in the first
quadrant. Find the maximum possible area of such a
rectangle. Justify your answer.
Solution: As shown in the video, our rectangle has
width x and height y, and so has area xy. But
y=4−x2, so our area is A(x)=x(4−x2)=4x−x3.
This
turns our word problem into just finding the maximum value
of A(x) on [0,2]. Since A′(x)=4−3x2, we have a
critical number when 4−3x2=0, or x=2√3. Then y=4−x2=4−43=83, and the area is A=xy=163√3=16√39.
(The video cuts out a few seconds too soon, but the end
of the calculation is shown to the left.)